I think there was a chapter that he mentioned he had a hunch.
*Grabs you by the pinky* TELL ME WHICH CHAPTER
I'm not really sure about this, but.
Remember when the top got into an argument with yumin about what he was writing/collection of intimate experiences with the top. The top was looking for him and he kind off took a bottle full of pink pill (heat inducers I think) before heading to yumin on his part-time acting job. After that, before they went to the cabin (where their staying now) Yumi was kinda looking for his heat inducers so that his ears won't puff out (he kinda said it on their first time that they shaboink, that he was thankful for the heat inducers or else his ears would've puffed out)
Interesting thank you so much. heh finally its revealed
Chapter 25. When he's talking to his sister
He was also hinted when the top found the heat reducers pills
So judging by that reaction he already knew he was a cat?